Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, a gas chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte.
A fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) or CO is supplied to the anode. The oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating DC electric energy.
In a fuel cell stack, for example, a fuel gas manifold extending in a direction of stacking the electrolyte electrode assemblies is provided for supplying the fuel gas through the fuel gas manifold to a fuel gas supply passage connected to the anodes of the electrolyte electrode assemblies. Likewise, the oxygen-containing gas is supplied to the cathodes of the electrolyte electrode assemblies through an oxygen-containing gas supply passage. Thus, the fuel gas supply passage and the oxygen-containing gas supply passage need to be sealed hermetically to prevent the mixture, leakage, or the like of the fuel gas and the oxygen-containing gas. Further, desirably, the seal member is an electrically insulating seal member, and thermally stable in the use at high temperature (operating temperature of about 800° C.).
In view of the above, Japanese Laid-Open Patent Publication No. 8-7902 discloses a flat plate type solid oxide fuel cell. Specifically, as shown in FIG. 25, a power generation cell 1 is sandwiched between current collecting plates (or separators) 2a, 2b. The power generation cell 1 includes a positive electrode 4a, a negative electrode 4b, and a ceramic thin membrane 3 interposed between the positive electrode 4a and the negative electrode 4b. Air supply grooves 5a for supplying the air to the positive electrode 4a are formed on the current collecting plate 2a, and fuel gas supply grooves 5b for supplying a fuel gas to the negative electrode 4b are formed on the current collecting plate 2b. 
A glassy first seal member 6a is interposed between outer edges of the current collecting plates 2a, 2b. Further, a glassy second seal member 6b is interposed between an outer end of the ceramic thin membrane 3 and the current collecting plate 2b inside the first seal member 6a. The first and second seal members 6a, 6b are melt at high temperature, and a pressure P is applied to the first and second seal members 6a, 6b to form a gas seal.
However, in the above conventional technique, the glassy first and second seal members 6a, 6b are provided at two positions for each of the unit cells. During operation of the fuel cell, the glass component may be shattered or volatilized. As a result, microcracks are generated in the glassy first and second seal members 6a, 6b. Therefore, the first and second seal members 6a, 6b may be degraded undesirably, and the structure is not economical. Further, since space is required for providing the first and second seal members 6a, 6b, the structure is not compact.